The technology disclosed herein refers to complex signal processing circuits which process a complex signal including an in-phase signal and a quadrature signal, and more particularly, to techniques of improving the accuracy of correction of a quadrature error and an amplitude error between an in-phase signal and a quadrature signal in complex signal processing circuits including an analog complex filter.
Conventionally, in the field of wireless communication, there is a technique called low-IF reception in which a radio frequency signal (RF signal) is downconverted into an intermediate frequency signal (IF signal) having a frequency in the vicinity of a baseband. In the low-IF reception, the IF signal has the reduced frequency and therefore the number of parts can be reduced, whereby the integration density of a receiver system can be increased, compared to a standard superheterodyne technique.
FIG. 8 shows a configuration of a conventional low-IF receiver circuit. A quadrature detector 93 multiplies a radio frequency signal RF which has been passed through a low-noise amplifier (LNA) 91 by a local signal LOi output from a local oscillator (LO) 92 to output an in-phase signal I90, and multiplies the radio frequency signal RF by a local signal LOq to output a quadrature signal Q90. Thus, the radio frequency signal RF is converted into an intermediate frequency signal (an analog complex signal (I90+jQ90) including the in-phase signal I90 and the quadrature signal Q90). Note that “j” is the imaginary unit. Note that, in the description that follows, a complex signal including an in-phase signal Ix and a quadrature signal Qx is represented as (Ix+jQx).
The radio frequency signal RF may contain an image component. The frequency of the image component and the frequency of a target component are located on the frequency axis at positions symmetrical with respect to the frequency of the local signal LOi. Therefore, the image component and the target component are mixed together due to frequency conversion performed by the quadrature detector 93, so that the image component can no longer be separated from the target component. Therefore, the image component needs to have been sufficiently attenuated in the analog complex signal (I90+jQ90). Such a technique of rejecting the image component is described in Japanese Patent Publication No. 2003-283354, etc.
Analog-to-digital converters (ADCs) 94i and 94q each convert the in-phase signal 190 and the quadrature signal Q90 into digital signals, i.e., an in-phase signal I92 and a quadrature signal Q92, respectively. An IQ imbalance correction circuit (IQ) 95 corrects a quadrature error and an amplitude error (IQ imbalance) between the in-phase signal I92 and the quadrature signal Q92, and outputs the results as an in-phase signal I93 and a quadrature signal Q93. A digital complex filter (DCF) 96 applies a complex operation to the in-phase signal I93 and the quadrature signal Q93 output from the IQ imbalance correction circuit 95 in order to reject the image component of the digital complex signal (I93+jQ93), and outputs the results as an in-phase signal I94 and a quadrature signal Q94, respectively. A digital signal processing circuit (DSP) 97 demodulates data based on the output of the digital complex filter 96.
By thus correcting the IQ imbalance using the IQ imbalance correction circuit 95, the image rejection ratio of the digital complex filter 96 can be improved. Such an IQ imbalance correction technique is described in Japanese Patent Publication Nos. H10-56484 and 2003-309612, etc.
When, however, an image rejection ratio desired for the analog complex signal (I90+jQ90) (i.e., the difference in image component signal level between the analog complex signal (I90+jQ90) and the digital complex signal (I94+jQ94)) is larger than the input range (i.e., the difference between the maximum input level and the noise level) of the analog-to-digital converters 94i and 94q, the analog-to-digital converters 94i and 94q cannot accurately convert the analog complex signal (I90+jQ90).
As shown in FIG. 9, therefore, an analog complex filter 90 for attenuating the image component of the analog complex signal (I90+jQ90) is provided at a stage preceding the analog-to-digital converters 94i and 94q. The analog complex filter 90 combines the in-phase signal I90 and the quadrature signal Q90 to output an in-phase signal I91 and a quadrature signal Q91.